1. Field of the Invention
The present invention relates to a manufacturing method of machine components as a composite member, in which the machine component is prepared as plural sections divided therefrom and they are united into the whole machine component, and more particularly to a manufacturing method of machine components, which is suited to manufacture of machine parts by forming one part composed of green compact and another part composed of green compact or sintered compact or wrought steel according to a powder metallurgical technique, assembining them and bonding them by sintering to complete the machine part as a composite member, especially manufacture of machine parts having complicated shapes or being required to partially have a specific property.
2. Related Art
Manufacture of a machine part having a complicated shape or a machine component partially having a specific property has difficulty in forming it as a single part. In such a case, generally, the machine part is once splited and formed in plural sections, and they are combined into one body to manufacture as a composite part. A few methods are known as the means for bonding plural sections into one body, and a proper bonding method may be selected as occasion requires to utilize for manufacturing those machine parts as a composite component. However, the manufacturing method of machine parts in accordance with powder metallurgy is exceedingly excellent for manufacture of composite components.
In the manufacturing method of composite components by powder metallurgy, at least one of the plural split sections is formed of green compact, and other sections are formed of green or sintered compact or wrought steel, and they are bonded together by contacting and sintering. At this time, in order to obtain a firm joint, a mutual tight contact of joint surfaces is required at the time of sintering, and the joint profile of the conventional composite components are simple to be sectioned and bonded in flat planes.
However, in manufacture of composite components, it is often required to compose the component in a complicated joint profile such that one portion is inserted into other portion. For manufacture of such composite parts, the manufacturing method in accordance with powder metallurgy is suitable, and various composite parts are manufactured by using plural sections formed by compact molding.
In manufacture of a sintered component by combining plural green compacts, there is a general case wherein one green compact is formed into a shaft portion, and other green compact is formed into a shape having a hole for accommodating the shaft portion, and the green compact having the shaft portion (hereinafter called the inner green compact because it is on the inner side when fitting) and the green compact having the hole (hereinafter called outer green compact) are sintered in the state the shaft portion is fitted in the hole so that they are combined into one body by diffusion bonding. In this case, it should be noted that the amount of thermal expansion of the green compact during sintering depends on the chemical composition thereof, and, in order to enhance the joint strength, the chemical compositions of the both green compacts are selected so that the thermal expansion of the outer green compact may be smaller than that of the inner green compact to achieve sintering in the tight contact of the both green compacts.
For example, Fe--Cu green compact which is easily expanded by sintering is used for the inner green compact, and Fe--Ni green compact which easily shrinks by sintering is used for the outer green compact. In this method, since the chemical composition partially differs in the whole component, only a component having partially different properties is obtained.
As a measure of solving this problem, the inventor of the present application has previously disclosed in Japanese Patent Publication No. (Kokoku) 62-35442 that, even when materials of common chemical composition are used in the inner portion and outer portion, the purpose of enhancement of joint strength is achieved by setting the carbon content of the inner portion 0.2% higher than that of the outer portion, so that the inner portion is relatively expanded more to achieve tight contact of the both portions and promote bonding by solid-phase diffusion of alloy components.
By this previous invention, alloys of similar compositions can be used in the outer portion and inner portion. However, since the carbon is an element having a large effect on properties of iron alloy, it is not preferred that its content differs between the outer portion and inner portion.
Besides, such sintered composite machine components are manufactured by bonding green compacts, but it may be necessary to manufacture composite parts using sintered compact or wrought steel in one part, depending on the application of components, function or other requirements.
In particular, welding of sintered alloy and other material is generally avoided because its porosity acts negatively, and if welding is necessary, it is forced to employ an expensive method such as laser welding by using filler wire of a special composition. Accordingly, when sintered alloy member and steel or other material are bonded without use of welding, both members were fixed by keys by drilling key grooves in both or tightened by bolts by drilling holes in both. As described above, manufacture of composite members using wrought steel in one part and a green compact in the other part is very useful because it can extend the area in which weldable machine parts are possibly manufactured.
However, bonding of steel material and green compact, in particular, bonding at complicated joint surfaces including insertion is not easy as explained below.
The green compact during sintering shows dimensional changes due to allotropic transformation and heat, which is same as in the case of molten material. However, since the green compact is densified (or shrinks) in the sintering process due to change of gaps between powder particles, i.e., closing to form pores and losing in the pores, the amount of thermal expansion in ordinary sintering is smaller in principle as compared with that of the steel material or the sintered material of the same composition.
Therefore, if the outer portion is green compact and the inner portion is steel or sinter compact, the outer portion (green compact) relatively must shrink more to contact tightly with the inner portion (steel, sinter) so that the both portions would be bonded sufficiently. In fact, in the case of bonding two green compacts, the diffusion of components of the green compacts is promoted and a high joint strength is obtained. In spite of this, in the case of the inner portion being made of steel or sintered compact, the required joint strength is not obtained in the ordinary sintering condition designed for mass production. Even in this case, the joint strength is possibly improved by changing the sintering conditions so as to sinter them at high temperature for a long time. However, such a change is difficult to put into practice from the aspects of production efficiency and cost.
As the countermeasure, there has been developed a bonding method in which the joint surface of the steel member is subjected to carburizing process before fitting with the green compact. This method makes use of the following phenomenon: that is, when a carburized layer of a carbon content higher than that of the green compact is formed on the steel surface, the expansion amount of the steel increases, and the diffusion of carbon from the carburized layer to the green compact is promoted sufficiently when sintering, and a high joint strength as in the case of two green compacts is obtained.
However, this method requires a long time of carburizing process by the ion carburizing method or the like, and the treating cost of steel material is high. Moreover, it cannot be applied to a material which is not suited to carburizing or an occasion where carburization is not preferred. Therefore, the scope of application is limited.
Further, in the case of the outer portion made of steel or sintered compact and the inner portion made of green compact, since the inner portion (green compact) relatively shrinks more to occure dissociation from the outer portion (steel, sintered compact), and bonding by sintering is more difficult than in the case mentioned above.
Besides, when bonding is made to sintered part to other part, application of welding had been avoided, because the sintered compact is not suited to welding with other members due to its porosity and other properties. Therefore, if a composite member using steel material is manufactured easily, it is possible to manufacture machine parts having both of the benefit of the sintered product and the benefit of the steel material which is suited to welding.